Click "Customize-Interior Modeling 2.0" and select the room to enter the modeling interface. The Extrude tool is to create a 3D solid by extruding a surface with a designated distance. Here is the instruction:
1 Select "Extrude" from the top toolbar
2 Click to select a surface.
3 Drag with the mouse to determine the extrusion distance and click again to complete the extrusion operation (you can also directly enter a value in the "Extrude-Distance" field below and hit "Enter" to finish the extrusion with the exact distance).
More Tips:
After selecting the surface, double click it and you could apply the last distance to extrude.
After selecting, press the "Alt" key (or ⌥ for mac) to switch between whether to keep the default extrusion surface.